In many applications, one or more low-cost, simple to use, isolated power supplies working from input voltages up to 100 volts are needed. Traditional solutions use flyback converters to generate this bias supply. Flyback designs typically utilize asymmetric transformer turn ratios for primary and secondary power windings, with an optocoupler and reference voltage, or an auxiliary winding for feedback regulation. Flyback converters require an elaborate compensation design to achieve stability. This results in a tedious design process and a bulky solution with a higher component count and cost.
Isolated buck converters, also known as Fly-Buck™ converters, combine a synchronous buck converter with coupled inductor windings to create isolated outputs. An isolated buck converter is created by replacing the output filter inductor of a synchronous buck converter with a coupled inductor, i.e., a flyback-type transformer, and rectifying the secondary winding voltage using a diode and a capacitor. FIG. 1 is a schematic circuit diagram representing an isolated buck converter. The isolated buck converter 100 of FIG. 1 includes a high-side transistor Q1 110, a low-side transistor Q2 115, a transformer 120 comprising a primary inductor winding 125 and a secondary inductor winding 130, a primary-side output capacitor Cout1 135, a secondary rectifier diode 140, and a secondary-side output capacitor Cout2 145. The duty cycle of the transistors Q1 110 and Q2 115 dictates the primary-side output voltage Vout1, and, in turn, the secondary-side output voltage Vout2. Feedback resistors RFB1 150 and RFB2 155 provide a feedback signal to a control circuit (not shown) that regulates the primary-side output voltage Vout1 by controlling the duty cycle of the transistors Q1 110 and Q2 115. Because the secondary-side output voltage Vout1 is related to the primary-side output voltage Vout1, this primary-side output voltage regulation also effects some degree of regulation of the secondary-side output voltage. However, due primarily to the leakage inductance of the primary-side inductor winding 125 and the secondary-side inductor winding 130 and the forward voltage drop of the rectifying diode 140, changes in the secondary-side output voltage Vout2 are not accurately reflected in the primary-side output voltage Vout1. Because the feedback loop only receives the primary-side output voltage information, the secondary-side voltage regulation degrades at lower input voltages Vin and/or higher output currents Iout2. So, for example, voltage droop that occurs on the secondary side is not seen on the primary side, and is not taken into account by the control circuit in setting the duty cycles of Q1 and Q2. Additional factors also contribute to the degradation of the secondary-side voltage regulation at lower input voltages Vin and/or higher output currents Iout2, such as the primary and secondary winding resistances.